Second-ranked Halep edged past Irina-Camelia Begu in an all-Romanian semifinal. Halep won 7-6 (2), 6-3 to reach her second final this year.
Halep is one win away from her 21st WTA title and second this year after she triumphed at the Dubai Championships in February, her last tournament before a five-month break due to the coronavirus pandemic.
Earlier Saturday, Begu had secured a semifinal spot after beating Sara Sorribes Tormo 6-2, 4-6, 6-2. Their quarterfinal was suspended on Friday due to rain with Begu leading 6-2, 0-1.
The 23rd-ranked Mertens defeated 69th-ranked Kristyna Pliskova 7-5, 7-6 (4) to reach her first final of the season. The Belgian was 3-0 down in the tiebreaker before staging a rally that she completed by converting her first match point when Pliskova’s return hit the net.
Mertens improved her record against the Czech to 3-0.
Pliskova added only two more aces to the 111 she had already hit this season while Mertens served seven in the match.
The Prague Open is the second European tournament since the WTA restarted amid the coronavirus pandemic, with strict health protocols in place.
In men's tennis, it was announced Saturday that a player tested positive for the coronavirus ahead of an ATP Challenger event in Prague. The ATP tour tweeted that “the player has entered mandatory quarantine” at his home in line with ATP and Czech health protocols.
It did not identify the player but said he was locally based and “was not a direct acceptance into either the main draw or qualifying events” of the tournament, scheduled to last until Aug. 22.